This invention relates generally to digital wireless mobile communications.
Code division multiple access (CDMA) for digital wireless mobile communications involves using correlation techniques to allow a receiver to decode one signal among many that are transmitted on the same carrier at the same time. Each user's signal includes a unique code that appears to be noise to all except the correct receiver. A channel in the code domain describes a combination of a carrier frequency and a code. CDMA generally starts out with a narrow band signal, which for full speech is 9600 bps. This signal is spread with the use of specialized codes to a bandwidth of 1.23 MHz. The ratio of the spread data rate to the initial data rate is called the processing gain.
Currently available cellular technology makes use of what is called second generation or “2G” technology. Initially, cellular telephone technology was implemented with Advanced Mobile Phone Systems (AMPS) which were analog. In about 1995, digital systems, such as CDMA, were introduced.
The global system for mobile communication (GSM) uses gaussian minimum shift keying (GMSK) modulation. GSM uses time division multiple access (TDMA) technology. Multiple users operate on the same radio channel simultaneously by sharing time slots. The GSM system allows eight mobile telephones to share a single 200 kHz bandwidth radio carrier channel for voice or data communications. For duplex operations, GSM voice communications are conducted on two 200 kHz wide carrier frequency channels.
The 200 kHz wide channels are called an absolute radio frequency channel numbers (ARFCN). The ARFCN denotes a forward and reverse channel pair, separated in frequency by 45 MHz, and each channel is time shared between as many as eight subscribers. Each time slot has a duration of 156.25 bits and occupies a time interval of 0.577 ms. Therefore, the transmission bit rate on each carrier to support eight physical channels is 156.25/0.577 or 270.8333 kbps using binary (BT=0.3) GMSK modulation. The effective channel transmission rate per user is 33.854 kbps (270.833 kbps/for 8 users).
A number of competing third generation or “3G” technologies are being debated within the industry at this time. The goal of the 3G technologies is to offer higher bit rate services. Such services may include multimedia, including video, Internet and electronic mail.
One standard for 3G technologies is the IMT-2000 standard which was propounded by the International Telecommunications Union (ITU). IMT stands for International Mobile Telecommunications and IMT-2000 is the name for Future Public Land Mobile Telecommunications Systems (FPLMTS). FPLMTS is targeted at developing mobile telecommunications systems to be used “anywhere-anyplace” around the year 2000 operating at approximately 2000 MHz.
In 3G technology, a “bit” is the fundamental information unit of input data. A “symbol” is a grouping of data bits based on modulation. Thus, a symbol arises after encoding but prior to spreading. A “chip” is the minimum bit period of the final spread data. “Channels” include physical channels that are transmitted in the air, defined by a frequency and code. A transport channel is defined by how the data is sent and logical channels are defined by the type of data.
The so-called wideband or W-CDMA technology has been proposed as the 3G solution by the European Telecommunications Standards Institute (ETSI) as their proposal to the ITU for IMT-2000. ETSI's proposal is identified as UTRA (Universal Mobile Telecommunication System Terrestrial Radio Access). (The standard can be found at www.itu.int/imt/2-radio_dev/proposals/index.html.)
Due to the proliferation of telephone standards and systems, it would be desirable to have a telephone which operates with more than one standard. However, conventionally, such phones are considered to be impractical because they generally require substantial duplication of the receiver and transmitter sections. This arises mainly due to the fact that the standards in many cases are so different that it is believed that separate electronics are necessary.
Thus, there is a continuing need for a dual mode phone which operates using more than one standard and enables a user to use the same telephone in areas which operate under different standards.